Quick response sprinklers utilizing frangible bulbs as the thermal responsive element are well-known. Typically, the thermal responsive element is mounted between the valve plug and an adjustment screw so as to axially pre-load the valve plug in sealing engagement with the outlet of the sprinkler body flow passage, and the axial (compressive) load on the frangible bulb is the same as the axial pre-load on the valve plug. In general, the response time of the sprinkler is directly proportional to the thickness of the bulb wall which, in turn, is directly proportional to the amount of axial load which the bulb must bear without breaking.
The axial load exerted on the bulb may be reduced, while retaining the desired pre-load on the valve plug, by distributing the axial load between the bulb and a support element also disposed between the adjustment screw and valve plug. The support element, however, may degrade performance of the thermal responsive element due to placement of the support element relative to the thermal responsive element. Thus, a stream of ambient air to be sensed by the thermal responsive element may impinge on the sprinkler at a particular angle. Depending on the angular orientation of the bulb and support element with respect to the sprinkler central axis, the bulb may be fully exposed to the stream of air or it may be partially or fully blocked from the stream by the support element. Although the bulb and support element may be initially configured to fully expose the bulb to the ambient stream of air when the sprinkler is first installed, vibration may cause the angular orientation of the valve assembly to shift so that the support element thermally blocks the bulb from the ambient air stream. Thermal obtrusion of the bulb by the support element, as heretofore described, is clearly undesirable as the thermal responsive element may fail to rupture at elevated air stream temperatures.
Various valve assembly configurations have been proposed either to distribute the axial preload or to introduce redundancy in the thermal responsive element. For example, German A.S. No. 1,065,276 discloses a sprinkler wherein at least two frangible bulbs and a metal peg are arranged in a triangular pattern between a pair of plates interposed between the adjustment screw and the valve plug. At least two frangible bulbs are required so that at least one will rupture (should the other fail) to release the valve plug at a predetermined elevated temperature.
U.S. Pat. No. 2,125,510 discloses an automatic sprinkler wherein the valve assembly includes a frangible bulb nested in a collapsible strut assembly. The axial load is equally distributed between the bulb and strut assembly.
U.K. patent No. 1,359,857 discloses an automatic sprinkler wherein the valve assembly includes a pair of thermal responsive elements, namely, a frangible bulb and a strut and explosive charge.
The problem solved by the present invention is that of reducing the axial load on the frangible bulb, whereby thinner walled, more lightweight and quicker acting bulbs may be utilized, without thermal obtrusion of the bulb by the support element at all angular orientations of the valve assembly with respect to its central axis.